To: File

From: Jon Hagen

Date: 9-17-99

Subject: New Collector - Electrical Design

The 430 MHz collector is the pickup structure which travels in the slotted waveguide, pulled or pushed by the carriage house. It diverts the two megawatts of power in the slotted waveguide into the turnstile junction in the carriage house. Topologically, the collector is a combination waveguide bend and twist. The original collector was designed and built in 1960 by the Special Products Division of the I-T-E Circuit Breaker Co. At installation, it was found to arc. Dielectric windows were improvised and the collector was pressurized with Freon - later sulfur hexaflouride. Maintenance of this gas system has been troublesome and costly. The old collector also has a mechanical problem - rapid wheel wear-out.

Goals for the new collector design are:

1. increased power handling capability to eliminate dielectric windows and sulfur hexaflouride pressurization.

2. a stronger wheel system than the original, which has the wheels splayed out at 45 degree angles to ride in the corners of the waveguide.

3. minimum leakage around the collector (blow-by).

4. minimum power reflected back into the slotted guide (matching).

5. maximum bandwidth (at least the 1MHz bandwidth of the transmitter but preferably more, so that dimensional tolerances won't be uncomfortably tight).

The Ansoft 3-d electromagnetic simulator, HFSS, was used at every step of the electrical design. The drawing "COLL011" shows the interior structure of the collector, which is a 19" x 5.25" input waveguide joined to a 21" x 5.25" output waveguide. (The output waveguide projects through the slotted waveguide). A 4" diameter x 5.25" post extends from floor to the ceiling in the input waveguide. It sits at one side of the output waveguide in order to break the collector's symmetry. Without this symmetry breaker, the incident power would be totally reflected back into the slotted guide. The insets at the front and back of the input waveguide are wheel wells. These allow the input waveguide to be 19" wide, which is farther from cutoff (14"), than the 16" width of the original collector. The wheel wells form an inductive iris. To cancel the reflection produced by this iris, a capacitive button (4" diameter x 1" high) is placed on the floor of the waveguide, midway between the wheel wells. The button reduces the wheel well reflection to -28.3 dB, as verified by running HFSS on the structure shown in the drawing "FENDERS." With the wheel well arrangement fixed, the precise position of the symmetry-breaking post and the distance to the back wall were adjusted to provide a match (-35.0 dB return loss) between the input and output waveguides. The maximum electric field strength inside the collector was found to be only about twice the maximum field strength in the input guide. Accordingly, the power handling capability of the collector should be about 25% that of the rectangular waveguide. Since 25% of 100MW is much greater than our 2.5MW, the new collector should need no sulfur hexaflouride.

The bandwidth of the collector is almost entirely limited by the symmetry-breaking post.

A frequency sweep of the structure "COLL011" produced transmission data which is shown below, together with 1962 measurements of the transmission through the old collector.

Frequency	Predicted	Measured

transmission of	transmission of

new collector	old collector

422 MHz	-.46 dB	-.43 dB

424	-.28	-.28

426	-.14	-.12

428	-.04	-.15

430	-.001	-0.04

432	-.06	-.12

434	-.23	-.28

436	-.56	-.52

Note that the band pass of the new design is no better than the old; the new design has as high a Q as the old design. Nevertheless, the simpler structure of the new design substantially reduces the magnitude of the maximum internal field.

The half-height (5.25") collector waveguide must be matched to the full-height (10.5") slotted waveguide. In designing the matching transformer it was assumed, for simplicity, that the input guide had no slot. (This seemed justified in that a butt joint between ordinary WR2100 guide and slotted WR2100 produces a reflection lower than -20dB). The transformer is simply a section of waveguide whose impedance is the geometric mean of the impedances, b/a, of WR2100 and the half-height collector body. The width of the transformer, a, is 19", the same as the width of the collector body. The height of the transformer, b, is therefore determined by the equation (b/19)2 = (10.5/21)(5.25/19). This yields b = 7.06". The length of the transformer is 9.93", one quarter of 39.72" which is the guide wavelength for a 19" wide waveguide at 430MHz. The transformer was simulated (see the drawing SCOOP ) and produced a return loss of -35.0 dB. The scoop was added to the collector (see the drawing COLL012) and the overall return loss was -26.7 dB. This simulation assumed a metal bulkhead in the plane where the transformer meets the WR2100 waveguide. In the final design, the gap is sealed by a two-section choke.

The front choke consists of pockets above and below the input scoop. These pockets have a width of 19" and a height of 0.47". The back short of these pockets is transformed to an open at the 3/4" slotted opening. This slot is a quarter wave from the nose of the scoop so, at the nose, the gap is effectively shorted. The second choke consists of pockets above and below the 5.25" waveguide which forms the body of the collector. The HFSS simulations predict that, without any chokes (see drawing CHOKE 111), the blow-by through the gap will be –9.8dB, i.e., about 10%. With only the front choke installed see drawing CHOKE, blow-by is reduced to -24.4 dB. With front and rear chokes installed blow-by is reduced to -52.4 dB.

The reflection coefficient of the finished collector will be measured and, if necessary, any residual reflection will be eliminated by installing an inductive post in the output waveguide.